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Neighbours who saw him on the roof of the Davidson, north Sydney, home with power tools over the weekend had no idea he was rigging up what has been described as an elaborate system to pump gas into the rooms.

Detectives are investigating the source of the poison found in gas canisters as family members in their home city of Bogota, Colombia, revealed the couple, whose son and daughter were autistic, was on the verge of divorce.

The fiercely protective mother, Maria Claudia Lutz, 43, was found dead in one room with their daughter Elissa, 11. Their son Martin, 10, was found in another room alone.

Engineer and technology expert Manrique, 44, was found on his own in another room. The family's german shepherd was also dead.

The bodies were discovered on Monday morning when staff at St Lucy's Catholic School alerted police after the children did not attend class and Lutz did not arrive to run the canteen.

Lutz was last seen on Friday after picking the children before having a coffee with other mothers.

BOC gas employees yesterday inspected the property, which was rigged throughout the roof to deliver the deadly fumes in what a police source described as an "extensive, elaborate and well planned" operation.

Police take away a laptop from the family home. Photo / Craig Greenhill, News Corp Australia

Police are leaning towards the fact that the Drake Business Logistics company executive acted alone but have not been able to rule out the possibility of a murder-suicide pact with his wife.

"We may never know whether she knew what was happening, we just don't know," a source said.

Police are also investigating "domestic tension" between the couple as relative's tributes on Facebook pointedly made no mention of Manrique.

A source close to Lutz, who wished to remain anonymous, has told the City Paper in Bogota that the relationship between the couple was "at times very tense due to the stress of bringing-up the children".

As the Colombian Consulate makes arrangements to fly relatives to Sydney to identify the bodies of Fernando Manrique, his wife Maria and their two children, Maria's grief-stricken sister Ana Lutz took to Facebook to pen her ­despair, writing: "No one can say anything different than you being a warrior, always fighting for everyone!

"My cute doll. One more angel in heaven, an angel given to their children, life!!"

The deaths of the couple and their children Elisa, 11, and Martin, 10, have left family and friends reeling as reports emerged that the family was gassed.